Nucleobases spontaneously aggregate in water by forming stacked dimers and multimers. It is assumed that the main contributions to the aggregation stem from hydrophobic and base-base dispersion interactions. By studying the uracil monomer and dimer in bulk water with the first principle molecular dynamics, we discuss dimer structure and provide evidence that stacking increases the uracil-water hydrogen bonding strength and alters the hydration structure of uracil. These changes have a significant influence on the intensity and shift of the carbonyl stretching band as revealed by simulated infrared absorption spectra of the monomer and dimer and available experimental spectra. The contributions of dipole-dipole, dispersion, and water mediated forces to the stacking are discussed. The reported findings are valuable for understanding the microscopic mechanism of heteroaromatic association in water which is relevant to a large range of chemical and biological systems.